A comparative heat inactivation study of indigenous microflora in beef with that of Listeria monocytogenes, Salmonella serotypes and Escherichia coli O157:H7

Aims: Thermal inactivation of a mixture of five strains of Listeria monocytogenes, four strains of Escherichia coli O157: H7 and eight serotypes of Salmonella were compared with that of indigenous microflora in 75% lean ground beef. Methods and Results: Inoculated meat was packaged in bags that were completely immersed in a circulating water bath and held at 55, 57.5 and 60 degreesC for predetermined lengths of time. The surviving cell population was enumerated by spiral plating heat-treated samples onto tryptic soya agar supplemented with 0.6% yeast extract and 1% sodium pyruvate. D-values, determined by linear regression, in beef were 77.49, 21.9, and 10.66 min at 55, 57.5, and 60 degreesC, respectively, for indigenous microflora ( z = 5.81 degreesC). When either of the three pathogens were heated in beef, their D-values calculated were significantly lower ( P < 0.05) than those of indigenous microflora at all temperatures. The slope of the thermal death time curve for L. monocytogenes, E. coli O157: H7 and indigenous microflora were similar. Using a survival model for nonlinear survival curves, the D-1-values at all temperatures for L. monocytogenes were significantly higher ( P < 0.05) compared with those for Salmonella serotypes, E. coli O157: H7 or indigenous microflora. However, higher recovery of a subpopulation of the indigenous microflora in beef exposed to heating at 55, 57.5 or 60 degreesC resulted in significantly higher ( P < 0.05) D-2-values at all three temperatures, compared with those of the three pathogens at the same test temperatures. Conclusions: If the thermal process is designed to ensure destruction of indigenous microbial flora, it should also provide an adequate degree of protection against L. monocytogenes, Salmonella serotypes or E. coli O157: H7. Significance and Impact of the Study: The results of this study will assist the retail food industry in designing acceptance limits on critical control points that ensure safety, without introducing pathogens in a retail food environment, against L. monocytogenes, E. coli O157: H7 and Salmonella in cooked ground beef.